Switching power supply regulators are widely used in the electronic arts, for example in battery management, because of their high efficiency. Such regulators convert a DC source at a reference voltage to a DC supply at a different voltage. The switching regulator uses an inductor, a transformer or a capacitor as an energy storage element to transfer energy from source to supply in discrete packets, defined by the ON and OFF times of a typically fixed frequency control signal CTL that controls the state of a switch. Feedback circuitry is typically used to maintain the supply voltage substantially constant, by adjusting the amount of ON time in accordance with load demands. One type of switching regulator is the buck regulator, which provides a DC supply voltage that is lower than the DC source voltage.
FIG. 1 is a diagram of an exemplary prior art buck switching regulator. In this regulator, an inductor L is the energy storage element. A capacitor C filters the output voltage VOUT, providing a more constant voltage over time. A reference supply having a voltage VSUPPLY is provided as an input. FIG. 2 is a diagram of current through inductor L over time. The following description of the operation of the regulator of FIG. 1 is made with reference also to FIG. 2. Control of the circuit is by a pulse width modulated (“PWM”) control signal.
In the ON phase, the control signal CTL from a control circuit 12 goes high, turning on transistor T1, and VSUPPLY is applied to the inductor L. At start-up, time t0, VOUT starts at essentially zero volts. Thus the full VSUPPLY is asserted across the inductor L, and current begins to flow through it, increasing over time. This causes an electric field to build in the inductor L, representing energy being stored. The current iL flowing through inductor L during the ON phase is available for the load, and the additional current over the load current begins charging capacitor C.
In the OFF phase, for example starting at time t1, CTL goes low, turning off transistor T1, and the supply voltage applied to the inductor L is removed. However, the current iL flowing in L cannot change instantaneously, so the voltage across L changes to maintain the current through L. Just as the current through L increased steadily during the ON, during the OFF phase it decreases steadily as the inductor field collapses. The current flowing through inductor L during the OFF phase is available for the load, and the additional current over the load current is available to charge capacitor C. The output voltage VOUT is provided to control circuit 12, which adjusts the relative ON and OFF times, i.e., the pulse width, of signal CTL to regulate the output voltage VOUT to a predetermined value VSS. Once VOUT is stable, assuming a constant load, the relative ON and OFF times remain substantially the same. However, as the load changes, the circuit responds to restore/maintain VOUT to its target value VSS.
Prior art regulators such as the one shown in FIG. 1 may not have the capability of detecting an overload or short circuit in the load. In such circumstances the output voltage drops below its average value during normal load conditions. The control circuit responds by increasing the ON time to supply more power in order to restore the output voltage to its target. However, supplying more power during a short circuit or overload condition may result in a catastrophic failure of the load circuit, and of the regulator circuit, as well
Some prior art regulators are provided with current limiting circuits, to prevent excessive current flow during overload conditions. For example, as shown in FIG. 1, a resistor R is placed in parallel with the switching transistor T1, which is switched in and out of the circuit by way of a second switching transistor T2 that is controlled by the same control signal CTL that controls transistor T1. The size of transistor T2 is much larger than that of transistor T1 so that its impedance when ON is substantially less than that of transistor T1. The voltage across transistor T1 during the ON phase is thus monitored by resistor R. This voltage is sensed and amplified by a gain amplifier 14, the output of which is provided to the non-inverting input of a comparator 16. The inverting input of comparator 16 is connected to a reference voltage VRef set to a value corresponding to an overcurrent condition of the regulator. The output of the comparator OCDet thus provides an indication of an overcurrent condition. Since overcurrent may occur in short spikes that are not damaging to the circuitry, the output of the comparator OCDet is filtered in a Deglitch circuit 18. The filtered output IDet of the Deglitch circuit 18 is used to determine if an overcurrent condition exists such that some measure should be taken to alleviate the overcurrent, such as the implementation of cycle skipping. In cycle skipping, the ON pulses of the control signal CTL are suppressed (skipped) for a preset number of cycles.
While cycle skipping provides some protection in overcurrent conditions, it has poor control on the output current in hard-short conditions, and the current may “run away,” i.e., increase until catastrophic failure occurs in the circuitry. On the other hand, setting the number of cycles to be skipped when overcurrent is detected to too large a number could prevent the current regulator to start up in a soft-short condition, although it would resolve the current run away problem.
Thus, it would be desirable to have a switching regulator having effective overcurrent control in hard short conditions, while still allowing startup in soft short conditions. In this regard, while an analog current limit might be proposed to solve the problem, such a solution suggests a closed-loop system, which would need to be stabilized, presenting further problems.